Gas-phase processes for the homopolymerization and copolymerization of monomers, especially olefin monomers, are well known in the art. Such processes can be conducted, for example, by introducing the gaseous monomer or monomers into a stirred and/or fluidized bed of resin particles and catalyst.
In the fluidized-bed polymerization of olefins, the polymerization is conducted in a fluidized-bed reactor, wherein a bed of polymer particles is maintained in a fluidized state by means of an ascending gas stream including gaseous reaction monomer. The polymerization of olefins in a stirred-bed reactor differs from polymerization in a gas fluidized-bed reactor by the action of a mechanical stirrer within the reaction zone, which contributes to fluidization of the bed. As used herein, the term “gas-phase reactor” will include fluidized-bed and stirred-bed reactors.
The start-up of a gas-phase reactor generally uses a bed of pre-formed polymer particles, i.e. a “seedbed.” After polymerization is initiated, the seedbed is sometimes referred to as a “reactor bed.”
The reactor bed includes a bed of polymer particles, catalyst(s), reactants and inert gases. This reaction mixture is maintained in a fluidized condition by the continuous upward flow of a fluidizing gas stream from the base of the reactor which includes recycle gas stream circulated from the top of the reactor, together with added make-up reactants and inert gases. A distributor plate is typically positioned in the lower portion of the reactor to help distribute the fluidizing gas to the reactor bed, and also to act as a support for the reactor bed when the supply of recycle gas is cut off. As fresh polymer is produced, polymer product is withdrawn to substantially maintain the height of the reactor bed. Product withdrawal is generally via one or more discharge outlets disposed in the lower portion of the reactor, near the distributor plate.
The polymerization process can employ Ziegler-Natta, metallocene or other known polymerization catalysts appropriate for the gas-phase process. A variety of gas phase polymerization processes are known. For example, the recycle stream can be cooled to a temperature below the dew point, resulting in condensing a portion of the recycle stream, as described in U.S. Pat. Nos. 4,543,399 and 4,588,790. This intentional introduction of a liquid into a recycle stream or directly into the reactor during the process is referred to generally as a “condensed mode” operation.
Further details of fluidized bed reactors and their operation are disclosed in, for example, U.S. Pat. Nos. 4,243,619, 4,543,399, 5,352,749, 5,436,304, 5,405,922, 5,462,999, and 6,218,484, the disclosures of which are incorporated herein by reference for the purposes of U.S. patent practice.
Sometimes during the production of olefin polymers in a commercial reactor, it is desirable or necessary to transition from one type of catalyst system producing polymers having certain properties and characteristics to another catalyst system capable of producing polymers of different chemical and/or physical attributes. Transitioning between compatible Ziegler-Natta type catalysts generally takes place easily. However, where the catalysts are incompatible or of different types, the process is typically complicated. For example, transitioning between a traditional Ziegler-Natta type catalyst and a chromium based catalyst (two incompatible catalysts), it has been found that some of the components of the traditional Ziegler-Natta catalysts or the cocatalyst/activator act as poisons to the chromium based catalyst. Consequently, these poisons inhibit the chromium catalyst from promoting polymerization.
In the past, to accomplish an effective transition between incompatible catalysts, the first catalyzed olefin polymerization process was terminated using various techniques known in the art. Then the reactor was purged and emptied. After a new seedbed was added but before new reactants were added, the reactor would undergo another purging step to remove any contaminants such as catalyst poisons, and water and/or oxygen that may have been introduced when emptying or refilling the reactor. Such purification steps were time consuming and costly, sometimes requiring about 4 days or more of reactor shutdown time before polymerization could be re-initiated in a commercial operation.
U.S. Pat. Nos. 5,442,019; 5,672,665; 5,753,786; and 5,747,612, each issued to Agapiou et al. have proposed methods for transitioning between two incompatible catalysts without halting the polymerization reaction and emptying the reactor to rid it of the original catalyst by (a) discontinuing the introduction of the first catalyst into the reactor, (b) introducing a catalyst killer, and (c) introducing a second catalyst into the reactor. However, having the polymer product from the first polymerization reaction present during the transition can result in product made from both catalysts, which can provide a final product with less than optimum polymer properties.
Publication document WO00/58377 by Bybee et al. (Bybee) discloses a process for transitioning between two incompatible polymerization catalysts by stopping the first polymerization reaction, removing the polymer in the reactor, purging the reactor with nitrogen, adding a seedbed of polymer particulates to the reactor and polymerizing olefins with a second polymerization catalyst. However, Bybee discloses opening the reactor during the step of removing the polymers from the first polymerization reaction, which allows contaminants such as moisture, air or other potential catalyst poisons to be introduced into the reactor. Moreover, by opening the reactor to atmospheric conditions, a thin layer of oxidized compounds can be formed on the reactor wall that can interfere with subsequent reactor operating continuity. Accordingly, Bybee requires a step of purging the reactor after the introduction of the seedbed to remove oxygen that has been introduced into the reactor. Bybee also discloses a step of adding a drying agent to the seedbed in the reactor to remove moisture that has been introduced as a result of opening the reactor. These purging and drying steps require additional reactor downtime, and equates to lost production and increased costs.
What is needed is a method for transitioning from one catalyst system to another catalyst system that is incompatible with the first catalyst system, with reduced gas-phase reactor down-time. The present invention satisfies this need.